1. Field of the Invention
The present invention relates to an image forming apparatus and method, and more particularly to an image forming apparatus and method having a developing unit using electrophotography and electrostatic recording.
2. Description of the Related Art
As an electrophotographic image forming apparatus that electrostatically transfers a toner image, which is electrostatically formed on the surface of a photoconductive body functioning as a supporting body, onto a recording material (such as paper) contacting the surface, an apparatus is known which utilizes a conductive transfer roller or corona electrification body as a transfer component. In the image forming apparatus, its transfer section is formed between the photoconductive body and transfer component by pressing or approximating the transfer component to the photoconductive body. The toner image on the photoconductive body is transferred onto the surface of the recording material by passing the recording material through the transfer section while supplying the transfer component with a transfer bias voltage opposite in polarity to the toner image on the photoconductive body.
As the photoconductive body used for the image forming apparatus, an organic photoconductive body (OPC photoconductive body) and an amorphous silicon photoconductive body (called “a-Si photoconductive body” from now on) are widely used. Among them, the a-Si photoconductive body has high surface hardness and high sensitivity to a semiconductor laser, and exhibits little deterioration caused by repeated use.
With such characteristics, the a-Si photoconductive body is used as an electrophotographic photoconductive body of a high-speed copying machine and laser beam printer (LBP). However, it has a variety of problems because it is produced through a process of transforming gas into plasma using high frequency or microwave, solidifying it, and forming a film by depositing it on an aluminum cylinder. More specifically, it is difficult to make the plasma uniform or to place the aluminum cylinder at the center of the plasma, and the film deposition conditions cannot be made uniform accurately all over the photoconductive body surface. Thus, a potential irregularity of about 20 volts occurs at developing locations all over the photoconductive body surface, and the potential irregularity offers a problem of causing density irregularity.
The potential irregularity is caused by: (1) the difference in charging ability because of the capacitance difference due to film thickness irregularity of the film deposition; and (2) the difference in potential attenuation characteristics caused by the local difference in the film quality because of the unevenness of the film deposition state.
Besides, using the a-Si photoconductive body brings about much larger post-charge potential attenuation than using the OPC photoconductive body even in a dark state. In addition, the potential attenuation is increased by an optical memory of image exposure. Accordingly, it is necessary to carry out pre-exposure before the charge to erase the optical memory due to the previous image exposure. The optical memory will be described here.
The image exposure after charging the a-Si photoconductive body will generate optical carriers, resulting in the potential attenuation. In this case, however, the a-Si photoconductive body has many dangling bonds (unbonded hands), which bring about a localized state that captures part of the optical carriers, thereby degrading their transit performance or reducing the recombination probability of the light-generating carriers. Accordingly, in the image forming process, part of the optical carriers generated by the exposure on the a-Si photoconductive body is released from the localized state simultaneously with the application of an electric field to the a-Si photoconductive body at the next step charging. Thus, the a-Si photoconductive body has a surface potential difference between the exposed section and the unexposed section, which constitutes the optical memory in the end.
Accordingly, it is common to erase the optical memory by making the optical carriers, which are latent within the a-Si photoconductive body, excessive and uniform all over the surface by carrying out uniform exposure with an exposure unit before charging. It is possible in this case to eliminate the optical memory (ghost) more effectively by increasing the light quantity of the pre-exposure emitted from a pre-exposure unit, or by bringing the wavelength of the pre-exposure closer to the spectral sensitivity peak of the a-Si photoconductive body (about 680-700 nm).
In this way, the optical memory can be erased by the pre-exposure. However, as described above, if the a-Si photoconductive body has the film thickness irregularity or the difference in the potential attenuation characteristics due to the film quality difference, electric fields applied between photoconductive layers change. This will cause a difference in the release of the optical carriers from the localized state, thereby bringing about potential irregularity at developing locations even if uniform charge is achieved at charging positions. In addition, as for the charging ability, since the capacitance becomes greater in such regions as the film thickness is reduced, it becomes disadvantageous, that is, as the charging ability reduces, the charging irregularity becomes conspicuous in the developing regions.
For these reasons, the potential attenuation becomes very large between the charging processing and developing processing, resulting in the potential attenuation of about 100 to 200 volts. As a result, the photoconductive body has a potential irregularity of about 10 to 20 volts all over its surface because of the foregoing film thickness irregularity and the difference in the potential attenuation characteristics. Since the a-Si photoconductive body, which has a large capacitance, has a lower contrast than the organic photoconductive body, the potential irregularity has a greater effect on the a-Si photoconductive body, thereby making the density irregularity more conspicuous. To solve these problems, the present inventor proposes an electrophotographic apparatus with a configuration that varies the exposure values in accordance with the potential attenuation characteristics of the image supporting body surface (see Japanese Patent Application Laid-open No. 2002-67387, for example).
The electrophotographic apparatus can provide good images without the density irregularity by correcting the potential attenuation characteristics of the image supporting body in the initial stage of the image supporting body. However, the potential attenuation characteristics of the image supporting body can vary over an extended period of use, thereby offering a problem of causing the density irregularity.
In addition, the initial characteristics of the apparatus can vary depending on its use environment, offering a problem of the density irregularity.
The present invention is implemented to solve the foregoing problems. It is therefore an object of the present invention to provide an image forming apparatus and method capable of forming good images without density irregularity even if the image supporting body varies with the passage of time.